Understanding electron transport systems of Streptomyces cytochrome P450

Structural characterization and fatty acid epoxidation of CYP184A1 from Streptomyces avermitilis

The genome of Streptomyces avermitilis contains 33 cytochrome P450 genes. Among the P450 gene products of S. avermitilis, we characterized the biochemical function and structural aspects of CYP184A1. Ultra-performance liquid chromatography-tandem mass spectrometry analysis showed that CYP184A1 induced an epoxidation reaction to produce 9,10-epoxystearic acid. Steady-state kinetic analysis yielded a k_cat value of 0.0067 min^-1 and a K_m value 10 μM. The analysis of its crystal structures illustrated that the overall CYP184A1 structure adopts the canonical scaffold of cytochrome P450 and possesses a narrow and deep substrate pocket architecture that is required for binding to linear chain fatty acids. In the structure of the CYP184A1 oleic acid complex (CYP184A1-OA), C9-C10 of oleic acid was bound to heme for the productive epoxidation reaction. This study elucidates the roles of P450 enzymes in the oxidative metabolism of fatty acids in Streptomyces species.

Reviewing a plethora of oxidative-type reactions catalyzed by whole cells of Streptomyces species

Selective oxidation reactions represent a challenging task for conventional organic chemistry. Whole-cell biocatalysis provides a very convenient, easy to apply method to carry out different selective oxidation reactions including chemo-, regio-, and enantio-selective reactions. Streptomyces species are important biocatalysts as they can catalyze these selective reactions very efficiently owing to the wide diversity of enzymes and enzymatic cascades in their cell niche. In this review, we present and analyze most of the examples reported to date of oxidative reactions catalyzed by Streptomyces species as whole-cell biocatalysts. We discuss 33 different Streptomyces species and strains and the role they play in different oxidative reactions over the past five decades. The oxidative reactions have been classified into seven categories that include: hydroxylation of steroids/non-steroids, asymmetric sulfoxidations, oxidation of aldehydes, multi-step oxidations, oxidative cleavage, and N-oxidations. The role played by Streptomyces species as recombinant hosts catalyzing bio-oxidations has also been highlighted.

Cytochrome P450 Enzymes and Drug Metabolism in Humans

Human cytochrome P450 (CYP) enzymes, as membrane-bound hemoproteins, play important roles in the detoxification of drugs, cellular metabolism, and homeostasis. In humans, almost 80% of oxidative metabolism and approximately 50% of the overall elimination of common clinical drugs can be attributed to one or more of the various CYPs, from the CYP families 1–3. In addition to the basic metabolic effects for elimination, CYPs are also capable of affecting drug responses by influencing drug action, safety, bioavailability, and drug resistance through metabolism, in both metabolic organs and local sites of action. Structures of CYPs have recently provided new insights into both understanding the mechanisms of drug metabolism and exploiting CYPs as drug targets. Genetic polymorphisms and epigenetic changes in CYP genes and environmental factors may be responsible for interethnic and interindividual variations in the therapeutic efficacy of drugs. In this review, we summarize and highlight the structural knowledge about CYPs and the major CYPs in drug metabolism. Additionally, genetic and epigenetic factors, as well as several intrinsic and extrinsic factors that contribute to interindividual variation in drug response are also reviewed, to reveal the multifarious and important roles of CYP-mediated metabolism and elimination in drug therapy.

Dinotefuran alters Collembola-fungi-bacteria interactions that control mineralization of maize and soil organic carbon

Rare studies investigated influence of neonicotinoid insecticides on the whole soil biota including non-target invertebrates and microorganisms. And less is known about the consequent intervention on soil C processes. This study aimed to decipher Collembola-fungi-bacteria interactive effects on pathways of maize C translocation, combining isotopic tracer analysis of relevant compartments with high-throughput sequencing for bacterial and fungal genetic profiles. Dinotefuran was applied at 0 or 100 μg kg^-1 (a simulating residual dosage) to microcosms containing soils, Collembola and ¹³C labelled maize. Dinotefuran drastically reduced the density and maize-derived biomass C of Collembola, while intensifying antagonistic associations between soil organisms, with flourishing growth of Ascomycota and Actinobacteria, e.g., Streptomyces. This led to higher soil organic C (SOC) mineralization (elevated by 9.8-10.5%) across soils, attributing to the shift in microbial taxonomic and functional guild, e.g., with the increased abundance of genes aligned to cytochrome P450. Maize decomposition was controlled by Collembola that primarily fed on maize, via grazing behavior that facilitated labile maize C preferred decomposers, e.g., Xanthomonadaceae. These findings elucidate the influence of minute dinotefuran on intra-linkages between biomes (Collembola, fungi and bacteria), and highlight such legacy effects on maize and SOC mineralization.

Hydroxylation, Epoxidation, and Dehydrogenation of Capsaicin by a Microbial Promiscuous Cytochrome P450 105D7

Cytochrome P450 enzymes (P450s) are versatile biocatalysts, which insert a molecular oxygen into inactivated C-H bonds under mild conditions. CYP105D7 from Streptomyces avermitilis has been reported as a bacterial substrate-promiscuous P450 which catalyzes the hydroxylation of 1-deoxypentalenic acid, diclofenac, naringenin, compactin and steroids. In this study, CYP105D7 catalyzes hydroxylation, epoxidation and dehydrogenation of capsaicin, a pharmaceutical agent, revealing its functional diversity. The kinetic parameters of the CYP105D7 oxidation of capsaicin were determined as K_m =311.60±87.30 μM and k_cat =2.01±0.33 min^-1 . In addition, we conducted molecular docking, mutagenesis and substrate binding analysis, indicating that Arg81 plays crucial role in the capsaicin binding and catalysis. To our best knowledge, this study presents the first report to illustrate that capsaicin can be catalyzed by prokaryotic P450s.

Production of a Human Metabolite of Atorvastatin by Bacterial CYP102A1 Peroxygenase

Atorvastatin is a widely used statin drug that prevents cardiovascular disease and treats hyperlipidemia. The major metabolites in humans are 2-OH and 4-OH atorvastatin, which are active metabolites known to show highly inhibiting effects on 3-hydroxy-3-methylglutaryl-CoA reductase activity. Producing the hydroxylated metabolites by biocatalysts using enzymes and whole-cell biotransformation is more desirable than chemical synthesis. It is more eco-friendly and can increase the yield of desired products. In this study, we have found an enzymatic strategy of P450 enzymes for highly efficient synthesis of the 4-OH atorvastatin, which is an expensive commercial product, by using bacterial CYP102A1 peroxygenase activity with hydrogen peroxide without NADPH. We obtained a set of CYP102A1 mutants with high catalytic activity toward atorvastatin using enzyme library generation, high-throughput screening of highly active mutants, and enzymatic characterization of the mutants. In the hydrogen peroxide supported reactions, a mutant, with nine changed amino acid residues compared to a wild-type among tested mutants, showed the highest catalytic activity of atorvastatin 4-hydroxylation (1.8 min−1). This result shows that CYP102A1 can catalyze atorvastatin 4-hydroxylation by peroxide-dependent oxidation with high catalytic activity. The advantages of CYP102A1 peroxygenase activity over NADPH-supported monooxygenase activity are discussed. Taken together, we suggest that the P450 peroxygenase activity can be used to produce drugs’ metabolites for further studies of their efficacy and safety.

Cytochromes P450 (P450s): A review of the class system with a focus on prokaryotic P450s

Cytochromes P450 (P450s) are a large superfamily of heme-containing monooxygenases. P450s are found in all Kingdoms of life and exhibit incredible diversity, both at sequence level and also on a biochemical basis. In the majority of cases, P450s can be assigned into one of ten classes based on their associated redox partners, domain architecture and cellular localization. Prokaryotic P450s now represent a large diverse collection of annotated/known enzymes, of which many have great potential biocatalytic potential. The self-sufficient P450 classes (Class VII/VIII) have been explored significantly over the past decade, with many annotated and biochemically characterized members. It is clear that the prokaryotic P450 world is expanding rapidly, as the number of published genomes and metagenome studies increases, and more P450 families are identified and annotated (CYP families).

Production, extraction and characterization of Chlorella vulgaris soluble polysaccharides and their applications in AgNPs biosynthesis and biostimulation of plant growth

Chlorella vulgaris, like a wide range of other microalgae, are able to grow mixotrophically. This maximizes its growth and production of polysaccharides (PS). The extracted polysaccharides have a complex monosaccharide composition (fructose, maltose, lactose and glucose), sulphate (210.65 ± 10.5 mg g-1 PS), uronic acids (171.97 ± 5.7 mg g-1 PS), total protein content (32.99 ± 2.1 mg g-1 PS), and total carbohydrate (495.44 ± 8.4 mg g-1 PS). Fourier Transform infrared spectroscopy (FT-IR) analysis of the extracted polysaccharides showed the presence of N-H, O-H, C-H, -CH3, >CH2, COO-1, S=O and the C=O functional groups. UV-Visible spectral analysis shows the presence of proteins, nucleic acids and chemical groups (ester, carbonyl, carboxyl and amine). Purified polysaccharides were light green in color and in a form of odorless powder. It was soluble in water but insoluble in other organic solvents. Thermogravimetric analysis demonstrates that Chlorella vulgaris soluble polysaccharide is thermostable until 240°C and degradation occurs in three distinct phases. Differential scanning calorimetry (DSC) analysis showed the characteristic exothermic transition of Chlorella vulgaris soluble polysaccharides with crystallization temperature peaks at 144.1°C, 162.3°C and 227.7°C. The X-ray diffractogram illustrated the semicrystalline nature of these polysaccharides. Silver nanoparticles (AgNPs) had been biosynthesized using a solution of Chlorella vulgaris soluble polysaccharides. The pale green color solution of soluble polysaccharides was turned brown when it was incubated for 24 hours with 100 mM silver nitrate in the dark, it showed peak maximum located at 430 nm. FT-IR analysis for the biosynthesized AgNPs reported the presence of carbonyl, -CH3, >CH2, C-H,-OH and -NH functional groups. Scanning and transmission electron microscopy show that AgNPs have spherical shape with an average particle size of 5.76. Energy-dispersive X-ray (EDX) analysis showed the dominance of silver. The biosynthesized silver nanoparticles were tested for its antimicrobial activity and have positive effects against Bacillus sp., Erwinia sp., Candida sp. Priming seeds of Triticum vulgare and Phaseolus vulgaris with polysaccharides solutions (3 and 5 mg mL-1) resulted in significant enhancement of seedling growth. Increased root length, leaf area, shoot length, photosynthetic pigments, protein content, carbohydrate content, fresh and dry biomass were observed, in addition these growth increments may be attributed to the increase of antioxidant activities.

Streptomyces Cytochrome P450 Enzymes and Their Roles in the Biosynthesis of Macrolide Therapeutic Agents

The study of the genus Streptomyces is of particular interest because it produces a wide array of clinically important bioactive molecules. The genomic sequencing of many Streptomyces species has revealed unusually large numbers of cytochrome P450 genes, which are involved in the biosynthesis of secondary metabolites. Many macrolide biosynthetic pathways are catalyzed by a series of enzymes in gene clusters including polyketide and non-ribosomal peptide synthesis. In general, Streptomyces P450 enzymes accelerate the final, post-polyketide synthesis steps to enhance the structural architecture of macrolide chemistry. In this review, we discuss the major Streptomyces P450 enzymes research focused on the biosynthetic processing of macrolide therapeutic agents, with an emphasis on their biochemical mechanisms and structural insights.

Streptomyces spp. in the biocatalysis toolbox

About 20,100 research publications dated 2000-2017 were recovered searching the PubMed and Web of Science databases for Streptomyces, which are the richest known source of bioactive molecules. However, these bacteria with versatile metabolism are powerful suppliers of biocatalytic tools (enzymes) for advanced biotechnological applications such as green chemical transformations and biopharmaceutical and biofuel production. The recent technological advances, especially in DNA sequencing coupled with computational tools for protein functional and structural prediction, and the improved access to microbial diversity enabled the easier access to enzymes and the ability to engineer them to suit a wider range of biotechnological processes. The major driver behind a dramatic increase in the utilization of biocatalysis is sustainable development and the shift toward bioeconomy that will, in accordance to the UN policy agenda "Bioeconomy to 2030," become a global effort in the near future. Streptomyces spp. already play a significant role among industrial microorganisms. The intention of this minireview is to highlight the presence of Streptomyces in the toolbox of biocatalysis and to give an overview of the most important advances in novel biocatalyst discovery and applications. Judging by the steady increase in a number of recent references (228 for the 2000-2017 period), it is clear that biocatalysts from Streptomyces spp. hold promises in terms of valuable properties and applicative industrial potential.

Cytochromes P450 for natural product biosynthesis in Streptomyces: sequence, structure, and function

Covering: up to January 2017Cytochrome P450 enzymes (P450s) are some of the most exquisite and versatile biocatalysts found in nature. In addition to their well-known roles in steroid biosynthesis and drug metabolism in humans, P450s are key players in natural product biosynthetic pathways. Natural products, the most chemically and structurally diverse small molecules known, require an extensive collection of P450s to accept and functionalize their unique scaffolds. In this review, we survey the current catalytic landscape of P450s within the Streptomyces genus, one of the most prolific producers of natural products, and comprehensively summarize the functionally characterized P450s from Streptomyces. A sequence similarity network of >8500 P450s revealed insights into the sequence-function relationships of these oxygen-dependent metalloenzymes. Although only ∼2.4% and <0.4% of streptomycete P450s have been functionally and structurally characterized, respectively, the study of streptomycete P450s involved in the biosynthesis of natural products has revealed their diverse roles in nature, expanded their catalytic repertoire, created structural and mechanistic paradigms, and exposed their potential for biomedical and biotechnological applications. Continued study of these remarkable enzymes will undoubtedly expose their true complement of chemical and biological capabilities.

Biosynthesis of Silver Nanoparticles by Streptomyces griseorubens isolated from Soil and Their Antioxidant Activity

Microbial mediated biological synthesis of metallic nanoparticles was carried out ecofriendly in the present study. Silver nanoparticles (AgNPs) were extracellularly biosynthesised from Streptomyces griseorubens AU2 and extensively characterised by ultraviolet-visible (UV-vis) and Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, scanning electron microscopy and X-ray diffraction analysis. Elemental analysis of nanoparticles was also carried out using energy dispersive X-ray spectroscopy. The biosynthesised AgNPs showed the characteristic absorption spectra in UV-vis at 422 nm which confirmed the presence of metallic AgNPs. According to the further characterisation analysis, the biosynthesised AgNPs were found to be spherical and crystalline particles with 5-20 nm average size. Antioxidant properties of the biosynthesised AgNPs were determined by 2,2-diphenyl-1-picrylhydrazyl free radical scavenging assay and was found to increase in a dose-dependent matter. The identification of the strain was determined by molecular characterisation method using 16s rDNA sequencing. The present study is the first report on the microbial biosynthesis of AgNPs using S. griseorubens isolated from soil and provides that the active biological components found in the cell-free culture supernatant of S. griseorubens AU2 enable the synthesis of AgNPs.

Exploring the electron transfer pathway in the oxidation of avermectin by CYP107Z13 in Streptomyces ahygroscopicus ZB01

Streptomyces ahygroscopicus ZB01 can effectively oxidize 4″-OH of avermectin to form 4″-oxo-avermectin. CYP107Z13 is responsible for this site-specific oxidation in ZB01. In the present study, we explored the electron transfer pathway in oxidation of avermectin by CYP107Z13 in ZB01. A putative [3Fe-4S] ferredoxin gene fd68 and two possible NADH-dependent ferredoxin reductase genes fdr18 and fdr28 were cloned from the genomic DNA of ZB01. fd68 gene disruption mutants showed no catalytic activity in oxidation of avermectin to form 4″-oxo-avermectin. To clarify whether FdR18 and FdR28 participate in the electron transfer during avermectin oxidation by CYP107Z13, two whole-cell biocatalytic systems were designed in E. coli BL21 (DE3), with one co-expressing CYP107Z13, Fd68 and FdR18 and the other co-expressing CYP107Z13, Fd68 and FdR28. Both of the two biocatalytic systems were found to be able to mediate the oxidation of avermectin to form 4″-oxo-avermectin. Thus, we propose an electron transfer pathway NADH→FdR18/FdR28→Fd68→CYP107Z13 for oxidation of avermectin to form 4″-oxo-avermectin in ZB01.

Cytochromes P450 as promising catalysts for biotechnological application: chances and limitations

Cytochromes P450 (CYPs) belong to the superfamily of heme b containing monooxygenases with currently more than 21,000 members. These enzymes accept a vast range of organic molecules and catalyze diverse reactions. These extraordinary capabilities of CYP systems that are unmet by other enzymes make them attractive for biotechnology. However, the complexity of these systems due to the need of electron transfer from nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) via redox partner proteins for the initial hydroxylation step limits a broader technical implementation of CYP enzymes. There have been several reviews during the past years tackling the potential CYPs for synthetic application. The aim of this review is to give a critical overview about possibilities and chances for application of these interesting catalysts as well as to discuss drawbacks and problems related to their use. Solutions to overcome these limitations will be demonstrated, and several selected examples of successful CYP applications under industrial conditions will be reviewed.

Microalgae associated Brevundimonas sp. MSK 4 as the nano particle synthesizing unit to produce antimicrobial silver nanoparticles

The biosynthesis of silver nanoparticles and its antimicrobial property was studied using bacteria isolated from Spirulina products. Isolated bacteria were identified as Bacillus sp. MSK 1 (JX495945), Staphylococcus sp. MSK 2 (JX495946), Bacillus sp. MSK 3 (JX495947) and Brevundimonas sp. MSK 4 (JX495948). Silver nanoparticles (AgNPs) were synthesized using bacterial culture filtrate with AgNO3. The initial syntheses of Ag nanoparticles were characterized by UV-vis spectrophotometer (by measuring the color change to intense brown). Fourier Transform Infrared Spectroscopy (FTIR) study showed evidence that proteins are possible reducing agents and Energy-dispersive X-ray (EDX) study showing the metal silver as major signal. The structure of AgNPs was determined by Scanning electron microscopy (SEM) and X-ray diffraction (XRD). Synthesized Ag nanoparticles with an average size of 40-65 nm have antimicrobial property against human pathogens like Proteus vulgaris, Salmonella typhi, Vibrio cholera, Streptococcus sp., Bacillus subtilis, Staphylococcus aureus, and Escherichia coli. Among the isolates Brevundimonas sp. MSK 4 alone showed good activity in both synthesis of AgNPs and antimicrobial activity. This work demonstrates the possible use of biological synthesized silver nanoparticles to combat the drug resistant problem.

Engineering of a hybrid biotransformation system for cytochrome P450sca-2 in Escherichia coli

P450sca-2 is an industrially important enzyme that stereoselectively converts mevastatin into pravastatin. However, little information or engineering efforts have been reported for this enzyme or its redox partner. In this study, we successfully reconstituted the P450sca-2 activity in Escherichia coli by co-expression with putidaredoxin reductase (Pdr) and putidaredoxin (Pdx) from the Pseudomonas putida cytochrome P450cam system. With an HPLC-based screening assay, random mutagenesis was applied to yield a mutant (R8-5C) with a pravastatin yield of the whole-cell biotransformation 4.1-fold that of the wild type. P450sca-2 wild-type and R8-5C were characterized in terms of mevastatin binding and hydroxylation, electron transfer, and circular dichroism spectroscopy. R8-5C showed an active P450 expression level that was 3.8-fold that of the wild type, with relatively smaller changes in the apparent k(cat)/K(M) with respect to the substrate mevastatin (1.3-fold) or Pdx (1.5-fold) compared with the wild type. Thus, the increase in the pravastatin yield of the whole-cell biotransformation primarily came from the improved active P450 expression, which has resulted largely from better heme incorporation, although none of the six mutations of R8-5C are located near the heme active site. These results will facilitate further engineering of this P450sca-2 system and provide useful clues for improving other hybrid P450 systems.

Application of a new versatile electron transfer system for cytochrome P450-based Escherichia coli whole-cell bioconversions

Cytochromes P450 monooxygenases are highly interesting biocatalysts for biotechnological applications, since they perform a diversity of reactions on a broad range of organic molecules. Nevertheless, the application of cytochromes P450 is limited compared to other enzymes mainly because of the necessity of a functional redox chain to transfer electrons from NAD(P)H to the monooxygenase. In this study, we established a novel robust redox chain based on adrenodoxin, which can deliver electrons to mitochondrial, bacterial and microsomal P450s. The natural membrane-associated reductase of adrenodoxin was replaced by the soluble Escherichia coli reductase. We could demonstrate for the first time that this reductase can transfer electrons to adrenodoxin. In the first step, the electron transfer properties and the potential of this new system were investigated in vitro, and in the second step, an efficient E. coli whole-cell system using CYP264A1 from Sorangium cellulosum So ce56 was developed. It could be demonstrated that this novel redox chain leads to an initial conversion rate of 55 μM/h, which was 52 % higher compared to the 36 μM/h of the redox chain containing adrenodoxin reductase. Moreover, we optimized the whole-cell biotransformation system by a detailed investigation of the effects of different media. Finally, we are able to demonstrate that the new system is generally applicable to other cytochromes P450 by combining it with the biotechnologically important steroid hydroxylase CYP106A2 from Bacillus megaterium.

Mangrove Streptomyces sp. BDUKAS10 as nanofactory for fabrication of bactericidal silver nanoparticles

Biosynthesis has led to the development of various biomimetic approaches for the fabrication of nanoscale materials. The present study reveals a unique procedure for the biosynthesis of bactericidal silver nanoparticles (AgNPs) using a novel Streptomyces sp. BDUKAS10, an isolate of mangrove sediment. Aqueous silver nitrate (AgNO(3)) solution was treated with cell free supernatant (CFS) of the isolate to synthesize bactericidal silver nanoparticles. Initial characterization was performed by visual observation for color change to intense brown color. UV-visible spectrophotometry (UV-vis) for measuring surface plasmon resonance indicated a maximum absorption peak at 441 nm. Fourier Transform Infrared Spectroscopy (FTIR) analysis provides evidence for proteins as possible reducing, and capping agents. Energy dispersive X-ray (EDAX) spectroscopy analysis showed elemental silver as major signal. Transmission Electron Microscopy (TEM) study indicated spherical silver nanoparticles in the size range of 21-48 nm. Compared to the CFS, the biosynthesized AgNPs exemplified superior bactericidal efficacy towards the tested bacterial strains. Results from this study suggested that Streptomyces sp. BDUKAS10 can be advantageous for the synthesis of AgNPs by extracellular method in the view of sustainable and ecofriendly approach.

Expression of CYP107Z13 in Streptomyces lividans TK54 catalyzes the oxidation of avermectin to 4″-oxo-avermectin

Streptomyces ahygroscopicus ZB01 has strong catalytic activity for the regiospecific oxidation of 4″-OH of avermectin to form 4″-oxo-avermectin. A cytochrome P450 gene from S. ahygroscopicus ZB01, cyp107z13, was cloned into pKC1139 to generate pKCZ1 and was transformed into Streptomyces lividans TK54, which does not have the ability to catalyze the conversion of avermectin. CYP107Z13, under the control of an ermE* promoter, was actively expressed in the TK54 recombinant strain as determined by a reduced CO difference spectrum analysis of the crude protein. Analysis of whole-cell biocatalytic activity by high-performance liquid chromatography revealed the recombinant to be able to oxidize avermectin regiospecifically to 4″-oxo-avermectin and CYP107Z13 to be a regioselective oxidase of avermectin. In addition, the whole-cell reaction conditions of the recombinant were optimized. Growth on medium ISP-2 at pH 6 was more conducive for the expression of CYP107Z13 than on medium PYG1 or at pH 7, and active cells of the recombinant strain had higher biocatalytic activity than resting cells.

Production and characterization of a functional putidaredoxin reductase-putidaredoxin covalent complex

In the cytochrome P450cam-dependent monooxygenase system from Pseudomonas putida, putidaredoxin (Pdx) shuttles electrons between putidaredoxin reductase (Pdr) and P450cam and, thus, must form transient complexes with both partners. 1-Ethyl 3-[3-(dimethylamino)propyl]carbodiimide (EDC) was found to promote formation of stoichiometric Pdr-Pdx complexes only when carboxyl groups on Pdx were activated. The yield of the EDC-mediated cross-link depended on the Pdx variant used and the redox state of both partners, decreasing in the following order: Pdr(ox)-Pdx(ox) > Pdr(ox)-Pdx(red) > or = Pdr(red)-Pdx(red). The Pdr-Pdx C73S/C85S conjugate was purified and characterized. Compared to the equimolar mixture of intact Pdr and Pdx, the fusion protein was more efficient in electron transfer to cytochrome c and, in the presence of saturating levels of P450cam, more effectively supported camphor hydroxylation. On the basis of our results, we conclude that (i) the cross-linked complex is physiologically relevant and represents a suitable model for mechanistic studies, (ii) molecular recognition between Pdr and Pdx is redox-controlled and assisted by the Glu72(Pdx)-Lys409(Pdr) charge-charge interactions, and (iii) the high specificity of the Pdr-Pdx couple may be due to finely tuned interactions at the protein-protein interface resulting in only one strongly preferred docking orientation leading to efficient FAD-to-[2Fe-2S] electron transfer.